The game length is primarily determined by (and balanced by) the number of event cards for the number of players.You could reduce the number of cards which shortens the game but I imagine that would make things tough.

Alternatively, just explore everything all the time to end the game quicker!

These are general things that should balance each other out, but I am not super experienced with the game so I can't give any exact numbers. You should make all 3 of these changes simultaneously to keep the game balanced.

A) Reduce the number of events in the game--will make the game harder on the players.B) Reduce the number of spaces on the exploration track or give players extra movement ("feet) for exploring--will make the game easier on the players.C) Reduce the number of markers needed to build the two big constructions (signal fire and sign in the sand)--will make the game easier on the players.

(Optional) Injuries and trauma might become less of an issue if you make the game shorter, so you might consider increasing the odds of an injury turning into a trauma at night.

I would suggest starting by reducing each of these elements by 1/3 and tweaking from there.